Photo Gallery: Nudibranchs

Spanish Dancer Nudibranch

Photograph by David Doubilet, National Geographic

There are more than 3,000 known nudibranch species, and scientists estimate there are another 3,000 yet to be discovered. So-called Spanish dancers, like this one off the coast of New South Wales, Australia, boast some distinctions over other nudibranchs: First, they can be enormous, reaching a foot and a half (46 centimeters) long. Most nudibranchs are finger-size. Second, it can swim, a skill most of its cousins lack.

Tritonia Nudibranch

Photograph by Jeffrey de Guzman, My Shot

Members of the mollusk family, nudibranchs abandoned their shells millions of years ago. Their scientific name, Nudibranchia, means "naked gills," and describes the feathery gills and horns that most, like this Tritonia species, wear on their backs.

Nudibranch, Philippines

Photograph by Libor Spacek, My Shot

Nudibranchs, nicknamed "nudis," are best known for the impossible array of colors and designs they sport. They derive coloring, as well as toxicity, from the food they eat. Their wild hues tell potential predators, "You'd best look elsewhere for a meal."

Nudibranch, Palau

Photograph by Ernie Collier, My Shot

Though partial to tropical climes, nudibranchs thrive throughout the oceans, in warm water and cold, from sandy shallows and reefs to the murky seabed a mile down. Chromodoris nudibranchs, like this one photographed near Palau, are generally a warm-water species.

Egg-Laying Nudibranch

Photograph by Jeffrey de Guzman, My Shot

Nudibranchs are hermaphroditic, carrying both male and female reproductive organs. Mating pairs fertilize one another and lay up to two million eggs in coils, ribbons, or tangled clumps, as this purple-painted Hypselodoris is doing.

Green-and-Orange Nudibranch

Photograph by Jeffrey de Guzman, My Shot

Nudibranchs are blind to their own beauty, their tiny eyes discerning little more than light and dark. Instead the animals smell, taste, and feel their world using head-mounted sensory appendages called rhinophores and oral tentacles.

Nudibranch Feeding

Photograph by David Doubilet, National Geographic

Nudibranchs are carnivores that slowly ply their range, grazing on algae, sponges, anemones, and corals. Many, like this red-white-and-blue aggressor devouring a sea hare near Bali, Indonesia, will assimilate the toxins and nematocysts (stinging cells) of their victims and use them for their own defense.

Sea Clown Nudibranch

Photograph by Minette Layne, My Shot

The toxic flesh of nudibranchs like this "sea clown" species is palatable to only a handful of sea creatures, including other nudibranchs. Other predators include certain fish, sea spiders, turtles, sea stars, a few crabs, and occasionally humans.

Multicolored Nudibranch

Photograph by Mohammed Alsaleh, My Shot

Generally oblong in shape, nudibranchs can be thick or flattened, long or short, ornately colored or drab to match their surroundings. Some max out at a quarter of an inch (6 millimeters), while others can reach a foot (30 centimeters) long or more.

Nudibranch, Bali

Photograph by David Doubilet, National Geographic

Nudibranchs' unique lives and body chemistry may harbor breakthroughs that could benefit mankind. Scientists are attempting to derive pharmaceuticals from their chemical armory and get clues to learning and memory from their simple nervous systems.

Nudibranch, Indonesia

Photograph by David Doubilet, National Geographic

A Ceratosoma nudibranch in Indonesia's Lembeh Strait hosts an imperial shrimp, who exchanges cleaning duties for a free ride and access to food.

Nudibranch, Philippines

Photograph by Clare Keating, My Shot

Nudibranchs' colorful lives are short, lasting less than a month in some cases, and rarely more than a year. Their elusive nature makes them difficult to study, and their boneless, shell-less bodies leave no record of their brief existence.

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